Once a year I hold a "parent/teacher" conference. I ask the parent to come to the lesson and we spend about 15 minutes of the lesson time (40 min. lessons) discussing their child's progress. Depending on the student, some participate in the conference, while others I may ask to have a private conversation with the parent.
We discuss many aspects of lessons.....effective practicing, evaluations, competitions, promptness, following direction, goals we've made as a student/teacher team, etc. I usually don't assign a grade. At this time, I always ask the parents for feedback about my teaching.....what they expect from me as a teacher and if they would like to offer any insights about their child. I have learned a lot from these conferences!
Monthly evaluations (just in paper form, not a conference) could be effective as well, but I would think quarterly or bi-annually is enough. Creating evaluation cards and completing them about each student in a thoughtful manner is very time consuming!
I hope this helps. If you have questions regarding this post, I will be out of town for a week, and won't be able to respond, but I will check when I return.